The present invention relates to an image forming apparatus such as a copying machine, a printer or the like using an electrophotographic process or electrostatic recording system, and more particularly to an image forming apparatus using a contact charging mechanism with electroconductive particles.
Heretofore, a corona discharging apparatus (corona type charging device) has been widely used as a charging apparatus for charging an image bearing member (member to be charged), for example, an electrophotographic photosensitive member, an electrostatically recordable dielectric member, or the like, in an image forming apparatus, for example, an electrophotographic apparatus, an electrostatic recording apparatus, or the like, to predetermined polarity and potential level.
A corona discharging apparatus is a noncontact type charging apparatus. It comprises an ion discharging electrode constituted of a piece of wire or the like, and an electrode in the form of a shield which surrounds the ion discharging electrode. The shield electrode is provided with an ion discharging opening directed toward the surface of an object to be charged, but, not in contact with the object. In operation, high voltage is applied to the ion discharging electrode and the shield electrode to generate discharge current (corona shower) to which the surface of the object is exposed to be charged to predetermined polarity and potential level.
In recent years, however, a substantial number of contact type charging apparatuses have been proposed, and some of them have been put to practical use as a charging apparatus because of their advantages over a corona type charging apparatus; for example, they are smaller in the amount of ozone production and power consumption.
A contact type charging apparatus comprises an electrically conductive charging member in the form of, for example, a roller (charge roller), a fur brush, a magnetic brush, or a blade, which is placed in contact with a member to be charged, for example, an image bearing member or the like. In operation, charge bias, or electrical voltage with a predetermined potential level, is applied to the contact charging member, which is placed in contact with a member to be charged, for example, an image bearing member or the like, so that the peripheral surface of the object to be charged is charged to predetermined polarity and electrical potential.
The charging mechanism (charging principle) in a contact type charging apparatus comprises a mixture of two charging mechanisms: (1) a mechanism based on electrical discharge, and (2) a mechanism based on injection of electrical charge. Thus, the characteristics of a contact type charging apparatus vary depending on which of the two mechanisms is dominant.
(1) Charging Mechanism Based on Electrical Discharge
This is a charging mechanism which charges the peripheral surface of an object to be charged, with the use of the products generated by the electrical discharge which occurs between a contact type charging member and the object to be charged.
In a charging system based on electrical discharge, there is a threshold value. Thus, in order for an object to be charged to a predetermined potential level, voltage, the potential level of which is greater than the predetermined voltage level, must be applied to a contact type charging member. In addition, an electrical discharge based charging system inherently produces by-products, the amount of which, however, may be drastically small compared to those produced by a corona based charging device. Therefore, even if a contact type charging system is employed, it is impossible to completely avoid the problems caused by active ions such as ozone.
(2) Mechanism Based on Electrical Charge Injection
This is a charging mechanism which directly injects electrical charge into an object from a contact charging member so that the peripheral surface of the object is electrically charged. It is called the charging system or a charge injection charging system. More specifically, a contact type charging member, the electrical resistance of which is in a medium range, is placed in contact with the peripheral surface of an object to be charged, to charge the object without triggering the electrical discharge. In other words, this charging mechanism is a charging mechanism which directly injects electrical charge into the peripheral surface of an object to be charged. Principally, it does not rely on electrical discharge. Therefore, even if the potential level of the voltage applied to a contact type charging member is less than a threshold voltage level, the object to be charged can be charged to a:potential level substantially equal to the potential level of the applied voltage.
Since this injection charging system does not involve ion generation, it does not suffer from the ill effects associated with the by-products of electrical discharge. However, since a contact type charging system is an injection system, its performance is greatly affected by the state of contact between a contact type charging member and an object to be charged. Thus, it is very important that a contact type charging member is high in density, that there is provided a sufficient amount of difference in surface velocity between the charging member and the object charged, and that the contact type charging member makes contact with the object to be charged, with a sufficiently high frequency.
A) Charging by Roller
A contact type charging apparatus which employs a roller type charging method, in other words, it employs an electrically conductive roller (charge roller) as a contact type charging member. It has been widely used because of its safety.
In the case of a charging roller, the charging mechanism based on electrical discharge (1) is the dominant charging mechanism.
A charge roller is formed of rubber or foamed material which is electrically conductive, or the electrical resistance of which is in the medium range. Sometimes, different materials are layered in order to obtain a predetermined characteristic.
A charge roller is provided with elasticity so that a predetermined state of contact can be kept between the charge roller and an object to be charged (hereinafter, photosensitive member). Therefore, a charger roller is given a large frictional resistance on its peripheral surface. Generally, it is enabled to follow the rotation of a photosensitive member, or is driven at a speed slightly different from that of the photosensitive member. Thus, when a charge roller is used to inject electrical charge into a photosensitive member, it cannot be avoided that the charge roller is deteriorated in its absolute performance and/or the state of contact between itself and the photosensitive drum by the contaminants adhered to the charge roller and/or the photosensitive member, and as a result, the photosensitive member is nonuniformly charged, in spite of the fact that a charge roller is a contact type charging member. In other words, in the case of a conventional charging roller, the charging mechanism based on electrical discharge is dominant in charging the photosensitive member.
FIG. 5 is a graph which shows the efficiencies of various contact type charging members. The abscissas represents the potential level of the bias applied to a contact type charging member, and the ordinate represents the correspondent potential level of a photosensitive member. The characteristic of a conventional charge roller is depicted by a line A. In other words, the charging of the photosensitive drum begins when the potential level of the voltage applied to the charge roller passes the threshold value of approximately xe2x88x92500 V. Therefore, generally, in order to charge a photosensitive drum to a potential level of xe2x88x92500 V, either a DC voltage of xe2x88x921,000 V is applied to the charge roller, or an AC voltage with a peak-to-peak voltage of 1,200 V is applied to the charge roller, in addition to a DC voltage of xe2x88x92500 V, so that a difference in potential level greater than the threshold voltage value is always present between the charge roller and the photosensitive drum, and the potential level of the photosensitive drum converges to the predetermined potential level, xe2x88x92500 V.
To describe in more detail, when a charge roller is placed in contact with a photosensitive drum with a 25 xcexcm thick photoconductor layer, the surface potential level of the photosensitive drum begins to rise as the potential level of the voltage applied to the charge roller is increased beyond approximately 640 V. Beyond 640 V, the surface potential level of the photosensitive drum linearly increases at an inclination of 1. This threshold potential level is defined as a charge initiation voltage Vth.
In other words, in order to increase the surface potential level of a photosensitive drum to a potential level of Vd, a DC voltage with a potential level of Vd+Vth, which is greater than the target surface potential level for the photosensitive drum, is necessary. This method in which only DC voltage is applied to a contact type charging member to charge an object is called a DC charge system.
However, it is rather difficult to change the value of the potential level of a photosensitive member to a desired value with the use of a DC charge system, because the resistance value of a contact type charging member varies due to changes in ambience, and also because the value of Vth changes as the thickness of the surface layer of the photosensitive member (image bearing member) changes as it is shaved.
Thus, various proposals to uniformly and reliably charge a photosensitive drum have been made. Among such proposals, U.S. Pat. No. 4,851,960 discloses an AC charge system, according to which a compound voltage composed of a DC voltage equivalent to a desired potential level and an oscillating AC voltage with a peak-to-peak voltage of 2xc3x97Vth is applied to a contact type charging member. This proposal intended that AC voltage be used to make the potential level uniform. As a result, the potential level of an object to be charged converges to the voltage value of Vd, the center of the top and bottom peaks of the AC voltage, which is not affected by external disturbance such as changes in ambience.
However, even in the case of such a contact type charging apparatus as the one described above, its charging mechanism principally relies on the electrical discharge. Therefore, the potential level of the voltage applied to a contact type charging member needs to have a value greater than the value of the potential level to which a photosensitive drum is to be charged. As a result, ozone is produced, although the amount is microscopic.
Further, when an AC charge system is used for the uniformity of charge, an additional amount of ozone is generated, and the contact type charging member and the photosensitive member are vibrated by the electric field generated by the AC voltage, which results in noises (AC charge noises). Further, the deterioration or the like of the peripheral surface of the photosensitive drum is very severe. These are new problems.
B) Charging by Fur Brush
In this charging method, a member with a brush portion formed of electrically conductive fibrous material is used as a contact type charging member (fur brush type charging device). In operation, the brush portion formed of electrically conductive fibrous material is placed in contact with a photosensitive member as an object to be charged, and charge bias with a predetermined potential level is applied to the brush portion to charge the peripheral surface of the photosensitive drum to predetermined polarity and potential level.
Also in the case of this fur brush type charging system, the dominant charging mechanism is the aforementioned charging mechanism based on electrical discharge (1).
There are two fur brush type charging devices which have been put to practical use: a fixed type, and a roller type. The former comprises a piece of pile composed by weaving fibrous material with an electrical resistance in an intermediary range, into base cloth, and attaching electrodes to the pile, whereas the latter comprises a metallic core and a piece of pile, similar to the one for the fixed fur brush type charging device, wrapped around the metallic core. As for the pile, those with a fiber density of approximately 100 strands/mmSUP2/SUP can be relatively obtainable. However, in order to charge a photosensitive member in a sufficiently uniform manner by the injection of electrical charge, such a fiber density is not high enough to maintain a satisfactory state of contact between the charging member and the photosensitive drum. Thus, it is necessary to provide between the peripheral surfaces of the charging member and photosensitive member such a velocity difference that is impossible to mechanically realize, which is not practical.
The characteristics of a fur brush type charging device when DC voltage is applied are depicted by a line B in FIG. 5. In other words, also in the case of a fur brush type charging device, whether it is of a fixed type or a roller type, a photosensitive drum is charged mostly through electrical charge generated by the application of charge bias with a potential level higher than the target potential level.
C) Charging by Magnetic Brush
In this charging method, a magnetic brush, that is, electrically conductive magnetic particles magnetically confined in the form of a brush on a magnetic roller or the like, is used as a contact type charging member. In operation, a magnetic brush is placed in contact with a photosensitive member, and charge bias with a predetermined potential level is applied to charge the peripheral surface of the photosensitive drum as an object to be charged, to predetermined polarity and potential level.
In the case of a magnetic brush type charging device, the dominant charging mechanism is the injection charging mechanism (2).
When electrically conductive magnetic particles ranging 5-50 xcexcm in diameter are used to form the magnetic brush, and a sufficient amount of difference in peripheral surface velocity is provided between the magnetic brush and a photosensitive drum, the photosensitive drum can be uniformly charged by the direct charge injection.
As is depicted by a line C in FIG. 5, that is, the graph which shows the characteristics of various types of charging devices, this magnetic brush type charging device can charge a photosensitive drum to a potential level substantially proportional to the potential level of the bias applied to a charging member.
However, this device also has its own problems. For example, it is complicated in structure, and some of the electrically conductive magnetic particles, of which the magnetic brush is composed, fall off and adhere to a photosensitive drum.
Japanese Patent Laid-Open Application No. 3,921/1994 or the like discloses a method for charging a photosensitive drum by directly injecting electrical charge into the charge retaining portions, for example, the trap levels or electrically conductive particles in the charge injection layer, of the photosensitive drum. This method does not rely on electrical discharge. Therefore, the potential level of the voltage to be applied to a charging member by this method has only to be as high as the potential level to which the photosensitive drum is charged, and also, it does not generate ozone. Further, it does not require the application of AC voltage. Therefore, there is no charging noise. In other words, this method is a superior charging method to a roller type charging method in that it does not produce ozone, and consumes a smaller amount of electrical power.
D) Toner Recycling System (Cleaner-less System)
In a transfer type image recording apparatus, the developing agent (toner) which remains on a photosensitive drum (image bearing member) after image transfer, or residual developing agent (residual toner), is removed from the peripheral surface of the photosensitive drum by a cleaner (cleaning apparatus) and becomes waste toner. From the standpoint of environmental protection, it is desired that waste toner is not produced. Thus, an image recording apparatus which employs a toner recycling system (or toner recycling process) has been realized. In this type of an image recording apparatus, there is no cleaner, and the residual toner which remains on a photosensitive drum after image transfer is removed from the photosensitive drum by a developing apparatus (developing-cleaning process). In other words, the residual toner is recovered by the developing apparatus.
The developing-cleaning process is a method in which the toner remaining on a photosensitive drum after image transfer is recovered by a fog removal bias (difference Vback between potential level of DC voltage applied to developing apparatus and potential level of peripheral surface of photosensitive drum) during the development of a latent image which follows image transfer, that is during the development after the next charging and exposure steps. According to this method, the residual toner is recovered by a developing apparatus and is used in the following image formation cycles. In other words, no toner is wasted; waste toner is not produced, reducing the amount of maintenance labor. Further, being cleaner-less makes a cleaner-less recording apparatus advantageous in terms of space; a cleaner-less recording apparatus can be drastically smaller compared to a recording apparatus with a cleaner.
In the clearnerless system, the residual toner is passed through a charging station and then a developing apparatus, instead of being removed from the peripheral surface of a photosensitive drum by a dedicated cleaner as described previously, so that it can be recycled to be used for the development processes in the following image formation cycles. Thus, a toner recycling system has its own problem, that is, how to properly charge a photosensitive member, with toner which is electrically insulative, being present in the contact portion between the photosensitive drum and a contact type charging member, since when a contact type charging member is employed as a means for charging a photosensitive member in a cleaner-less recording apparatus, the residual toner is definitely present between the photosensitive drum and the contact type charging member. When a photosensitive member is charged by a roller type charging member or a fur brush, the residual toner on the photosensitive drum is evenly scattered to remove the patterns in which the residual toner was distributed, and the photosensitive drum is charged mostly through the electrical discharge caused by the application of relatively large bias. When a magnetic brush is used to charge a photosensitive member, a brush portion composed of electrically conductive magnetic particles, that is, powder, flexibly contacts the photosensitive drum to charge it.
E) Coating of the contact charging member with particles
a. Japanese Patent Application Publication No. HEI 7-99442 discloses that in order to accomplish stabilized uniform charging without applying a charge directly from a contact charging device, charged particles are applied on the surface of a contact charging member contactable to the surface of the member to be charged. The contact charging member charging roller is driven by the member to be charged (photosensitive member) and although the amount of the ozone product is remarkably smaller than with the corona charger such as scorotron or the like, the main charging principle is still a discharge type charging mechanism similar to the above-described roller charging. Particularly, when the AC-biased DC voltage is used in order to provide more stabilized uniform charging, the amount of the ozone product due to the electric discharge increases. When the device is used in a long term, or when the cleanerless type image forming apparatus is used in the long-term, the problem arising from the ozone product such as image flow of the like tends to be remarkable.
Japanese Laid-open Patent Application No. HEI 5-150539 discloses an image forming method using contact charging wherein a developer contains at least visualizing particles and electroconductive particles having an average particle size small amount that of the visualizing particles in order to prevent charging defects caused by deposition of the toner particles and/or fine silica particle on the surface of the charging means with repeated image formation in long term. However, the contact charging still uses the discharge type charging mechanism, and therefore, it involves the above described problems.
b. U.S. application Ser. Nos. 09/035,109, 09/035,108, and 09/035,022 disclose that in order to promote the injection charging by an improvement in the contact property with the provision of a peripheral speed difference provided between the member to be charged and the contact charging member, a contact charging member is coated with electroconductive particles (charging-promotion particle), by which the close contactness is accomplished, and therefore, improper charging attributable to insufficient contact is eliminated.
Since the charging property is increased by the application of the charging-promotion particles on the contact charging member, the main function of the contact charging member is to provide a nip relative to the member to be charged and to support (carry) the charging-promotion particles, and the function of the contact charging member is carried out by the charging-promotion particles existing in the nip. Here, the conventionally called xe2x80x9ccontact charging member in such a system is called xe2x80x9csupporting member for charging-promotion particlexe2x80x9d (charging-promotion particle supporting member).
In a system in which the charging-promotion particles are applied on the supporting member for the charging particles in the injection charging mechanism using the charging-promotion particles in said E) -b, the amount of the charging-promotion particles carried on the surface of the supporting member for the charging-promotion particles decreases only by application of the charging-promotion particles on the charging-promotion particle supporting member at the initial stage, with a result of deterioration of the charging performance.
Therefore, means for supplying the charging-promotion particles to the charging-promotion particle supporting member is needed. As for such supplying means, a system in which the charging-promotion particles are supplied to the charging portion which is the nip between the image bearing member and the supporting member for the charging-promotion particle by way of the surface of the image bearing member (member to be charged) from the developing device, is advantageous since the developing device can be used as the supplying means for the charging-promotion particles so that downsizing is possible.
In the injection charging mechanism using the charging-promotion particles, the charging-promotion particles function as the contact charging member in effect, and therefore, the system supplying the charging-promotion particles from the developing device to the charging portion, is a new system in the charging-promotion particles which are virtually a contact charging member are always supplied from the developing device.
In such a system wherein the charging-promotion particles which are a contact charging member are supplied from the developing device into the charging portion which is a nip formed between the image bearing member and the charging-promotion particle, it is desirable that charging-promotion particles are supplied to the supporting member for the charging-promotion particles without non-uniformity in the longitudinal direction. When the charging-promotion particles are not supplied with stability, the distribution of the charging-promotion particles are not uniform on the surface of the supporting member for the charging-promotion particles. If not, the charging performance may be locally deteriorated.
However, when the charging-promotion particles are supplied from the developing device to the image bearing member using an electric field, the charging-promotion particles are supplied correspondingly to an image pattern during image formation. Therefore, non-uniform distribution of the charging-promotion particles may result on the surface of the charging-promotion particle supporting member.
It is a principal object of the present intention to provide an image forming apparatus wherein electric charge is injected into an image bearing member through electroconductive particles.
It is another object of the present intention to provide an image forming apparatus wherein an amount of electroconductive particles on a surface of the charging member is always enough.
It is a further object of the present intention to provide an image forming apparatus wherein local defficiency of the amount of the electroconductive particles is suppressed.
According to an aspect of the present intention, there is provided an image forming apparatus, includes an image bearing member; charging means for electrically charging said image bearing member, said charging means carrying electroconductive particles and having a charging member elastially press-contacted to said image bearing member; image forming means for forming an electrostatic image by selectively dissipating electric charge on said image bearing member charged by said charging means; developing means for developing the electrostatic image on said image bearing member with toner and for supplying the electroconductive particles to said image bearing member, wherein the electroconductive particles supplied by said developing means is carried to a press-contact portion of said charging member to contribution for electric charging of said image bearing member; and changing means for changing a relation between said charging member and a supply position of the electroconductive particles of said developing means.